/Engine/Plugins/Enterprise/DatasmithImporter/Source/DatasmithImporter/Public/UVTools/UVGenerationUtils.h
Returns the index of the next open UV channel, existing channels filled with default values are considered open. If no open channel is found, returns -1.
static int32 GetNextOpenUVChannel ( [UStaticMesh](API\Runtime\Engine\Engine\UStaticMesh) * StaticMesh, int32 LODIndex )